When attempting to unmount or delete a VMFS datastore on a cluster, you may get the message that the resource is still in use, even though all the VMs have been moved off the datastore, or that Storage I/O Control (SIOC) is enabled on it (usually when attempting to unmount).

To circumvent the issue, temporarily stop SIOC, unmount/delete the datastore, then start SIOC again. If an unplanned Permanent Device Loss (PDL) is invoked when Storage IO control enabled on a storage device (iSCSI device or FC), the ESX host cannot remount the VMFS datastore, and also the steps below are then also required in order to be able to remount the datastore.